What are the differences between a small and block and big block engine wiring harness for a 70's D100 truck?

I am aware that the big block is a little longer and things like senders might have a different location. That said, could I make a small block harness work on a big block or is it not worth the hassle?

The engine harness on my D100 has seen better days and has suffered a lot of amateur butchery. Rather than trying to refurbish the existing one with all the brittle connectors and corroded terminals, I figured I'd buy new. Just having a tough time locating a big block one for a reasonable price.

Not interested in rewiring the car with a universal kit or anything. The rest of the wiring is mostly OK, just the engine harness needs to be replaced.

Thanks for any input.

- Greg

the 2 wire zigzag dist connector would be on the opposite end as would the coil. the oil psi sender would (both) be on the top rear of the block (as you know). there'd be water temp sender location wherever it is at on the two diff engines. Just consider this a BTT for ya

i replaced a small block with a 440 in a 72 w100 many, many moons ago and used almost all of the small block wiring harness. i remember running the alternator harness under the intake, and moving the main harness on the firewall around some where it attaches with the fold over tabs. i also used the mount isolators and the chassis mounts from the small block combined with the block mounts from the donor engine [70 new yorker] along with pieces of 3/8 or 1/2" plate to adapt to the small block chassis mounts. the engine was located by connecting the original transmission mount [this had the divorced transfer case], then looking to see what i had to do to connect the engine to the chassis. i even used the new yorker pan with an extended sump made from another pan, as the proper truck pan was not available. it worked out great ! in my application, i was also able to use the hp manifolds from a super bee with the only mod needed being a very slight elongated, shallow "c" cut on the upper frame flange on the passenger side. of course, the pipes were custom bent by an exhaust shop. i know this is way more info than needed, but the wiring harness needed almost nothing changed to be used on the big block.

I done it with /6 harness and cut/splice/solder/heatshrink the connection for a clean looking install.

done a few mid 80s trucks swapping in /6-SB-BB engines.

I have unwrapped an 85 harness and removed all but a few needed wires to run my 88 440 truck bypassing the stock IGN. and BR along with any emissions crap under hood.

on a 70s it should be 3 plug at the firewall correct?

remove/unwrap it and remove what you don't want/need and cut/splice/solder/heatshrink it up with new wire.

I have cut up a bunch of truck wire harness so I have a box of correct color/long pieces to patch with.

I would look at a /6 harness as it will have most of the connections toward the front like a BB

or find another harness to cut up for patches for a clean up what you got now.

should be ez pz to mod it to work.

I swapped 3 harness out in my 79 RC/JY truck in about an hour time. removed 2 full 3 piece harness and installed 1.

Slant six harness is the closest to a big block, done is myself before.

Thanks. My coil is on the rear of the intake. Distributor wires might be an issue I forgot about that. I can get one of the electronic ignition harnesses and use that but hoping I won't have to do any splicing. Have aftermarket gauges for water temp/oil so I might not even use those parts of the harness.

After I posted this, I remembered that my truck was originally a small block. Based on the way some of the things were executed, it's hard to imagine the guy went out and got the correct big block harness.

Found a NOS harness for $92. Seemed like a good deal to me even if I have to modify it a little.

for a NOS harness..I would agree that's a great deal even if you have to mod it some.

I would try to cut pieces out of the old harness to lengthen wires as needed to keep it color coded. I like to solder connections then heat shrink them up good.

have you done the amp gauge bypass? that's the big problem on those yrs. heater circuit as well, it likes to melt the fusebox.

if not at least have a look at the wires/back of gauge cluster to look for burnt/melted plastic.

good luck.

I thought about doing the ammeter bypass. Someone had kind of done it already. They cut the red wire behind the bulkhead and spliced in a new one from the gauge lug and ran it through the firewall. I have a good factory wire from my Duster with the original connectors still in tact so I can re-use that.

I did also get an NOS bulkhead connector and starter relay so I might just put it back together like factory for now. Most of the time the connection in the bulkhead is what fails so if it's all new and clean, it should be fine for the time being. I can clean all the original terminals and pack 'em with dielectric grease.

Things look mostly OK under the dash, no melted stuff that I can see. There are some quick splices for the gauge wiring and the radio harness is hacked up pretty good for the cheapo speakers behind the seat. Someone unwrapped the whole thing to get to what they needed so the harness flops around which is annoying, will fix that when I do the bulkhead.

not trying to steal rmchrgr's post but has anyone ever used one of ron francis harness kits for a d100?

So the NOS harness I got is exactly the same as the one that was in the truck and fits perfect. The distributor wires are the right length, no splicing required anywhere. It's really nice to have new wiring!

I also switched out the original bulkhead connector and cleaned up the other harness connectors, there was a lot of corrosion and old, dried up dielectric grease.

Now I need to figure out what I am doing with the ammeter and then re-wrap the harness under the dash that the P.O. unwrapped for some reason. Fun times.

Thanks again for the replies.

you can wire in a volt meter

or

bolt/shrink wrap the 2 stock amp gauge wires together with a brass nut/bolt.

remember you just cant hook those 2 wires up to a VOLT gauge it will be wired different. DAMHIK!

IIRC..look for the MAD ELECTRICAL write up on amp gauges fixes.

this is the best thing to do for them.